1. Field of the Invention
The present invention relates to polymeric compositions with barrier properties, more particularly oxygen-impervious polymeric compositions containing semiaromatic polyamides. It also relates to packaging materials, especially for beverages and food products, made from these compositions.
2. Discussion of Technology
The use of polymers with barrier properties is at present practised on a vast scale in the manufacture of plastic packaging materials, chiefly for packaging beverages and foodstuffs.
Polymers which form a physical barrier to gases, especially to oxygen, and which therefore make it possible to lengthen the preservation and storage life of the packaged products without deterioration of the latter, are chiefly copolymers of ethylene and vinyl alcohol, vinylidene chloride copolymers and the polyamides resulting from the polycondensation of m-xylylenediamine with adipic acid (MXD6).
For cost reasons and/or for practical reasons (for example to impart other desired properties to the packaging material), it is often necessary to use these polymers in combination with other polymers such as, for example, ethylene, propylene and vinyl chloride polymers and polyalkylene terephthalates. These combinations are embodied in the presence, in the packaging material formed, of a plurality of layers of the chosen polymers with optional interposition of suitable adhesives. Typical structures of such materials in which the barrier polymer is MXD6 and the other polymer polyethylene terephthalate (PET) have been described, for example, in Patent Applications EP-A-0,161,625 and EP-A-0,186,154 (Mitsubishi Gas Chemical) and are particulary applicable to the manufacture of containers (bottles, flasks, etc.).
Containers of this type, in which an impervious layer consisting of a mixture of PET and of barrier polymer (MXD6) is interposed between two PET-based layers, especially to improve the adhesiveness between the layers after the biaxial orientation produced by the blow-moulding of the preformed parison, have been described in document EP-B-0,092,979 (Yoshino Kogyosho).
For reasons of cost and of final appearance of the packaging material, the MXD6 content of these mixtures is generally lower than 10% by weight.
To enable the MXD6 content to be decreased further while simultaneously improving the imperviousness to oxygen it has been proposed to incorporate into the MXD6 a carboxylate of a metal such as cobalt [Patent Application EP-A-0,301,719 (MB Group)]. It is assumed that the cobalt catalyses the oxidation of the MXD6, adding an effect of a "chemical barrier" to oxygen to the physical barrier consisting of the mixture of polymers (Revue des Industries Agro-Alimentaires, issue for 9 to 23 Apr. 1990, pages 34 and 35; La Recherche, volume 21, no. 222, June 1990, page 752).
This solution has not been found completely satisfactory because the MXD6 in which the cobalt carboxylate is incorporated contains phosphorus compounds. These compounds are incorporated during the polymerisation [see, for example, Patent Application DE-A-2,341,895 (Toyobo); Japanese Patent Applications (Kokai) published under numbers 74/53,945 (Toyobo) and 78/125,460 (Toyobo); Patent Application EP-A-0,361,636 (Amoco Corp.)] and/or added during the stabilisation of the MXD6 [see, for example, Patent Application DE-A-1,570,609 (Bayer AG); Patent GB-A-1,146,157 (Du Pont); Japanese Patent Application (Kokoku) published under number 73/20220 (Toyo Spinning); and Japanese Patent Application (Kokai) published under number 84/87,132 (Mitsubishi Chemical)]. These phosphorus compounds are generally chosen from (hypo)phosphorous, (hypo)phosphoric, phosphinic and phosphonic acids, their salts and esters and, in most cases, from sodium, manganese, calcium and zinc (hypo)phosphites. A phosphorus co pound which is frequently employed is sodium hypophosphite.
As a general rule, these phosphorus compounds can be found in MXD6, present in the abovementioned packaging materials, in concentrations which, expressed as phosphorus, can reach 500 ppm, in most cases between 150 and 400 ppm. For example, the product Reny 6001, which is the MXD6 typically employed for manufacturing the packaging materials according to Patent Application EP-A-0,301,719 (page 9, lines 10 to 13) contains 200 to 350 ppm of phosphorus.
This relatively plentiful presence of the phosphorus compound in the MXD6 is a source of disadvantages. It interferes with the "chemical barrier" function performed by the oxidation catalyst based on cobalt carboxylate, with the result that the latter, costly and ecologically dubious, has to be incorporated into the MXD6 in relatively large quantities. Furthermore, when the phosphorus compound is a reducing agent, as is frequently the case, and when the MXD6 is combined with PET to manufacture the packaging material, interfering reactions between the phosphorus compound and the catalyst residues from the polycondensation of PET (particularly organic antimony salts) give rise to discolorations in the packaging material which are particularly unacceptable in the case where articles intended for the packaging of liquids (sparkling beverages) are involved.
Finally, bearing in mind the antagonist effects exerted by the phosphorus compound and by the cobalt carboxylate on the oxidation of MXD6, a long delay, called an "induction period" hereinafter, may elapse between the time when the packaging material is manufactured and the time when its imperviousness to oxygen has reached an acceptable value. During this induction period, which may, for example, reach 30 days (Patent Application EP-A-0,301,719, page 8, lines 7 to 9), the initial oxygen permeability gradually decreases. This disadvantage is counteracted by costly (aging) techniques or by incorporating more oxidation catalyst (ibid., page 8, lines 2 to 7).